Ion implantation has become the technology preferred by industry to dope semiconductor wafers with impurities in the large scale manufacture of integrated circuits. Ion dose and ion energy are the two most important variables used to define an implant step. Ion dose relates to the concentration of implanted ions for a given semiconductor material. Typically, high current implanters (generally greater than 10 milliamps (mA) ion beam current) are used for high dose implants, while medium current implanters (generally capable of up to about 1 mA beam current) are used for lower dose applications.
Ion energy is used to control junction depth in semiconductor devices. The energy levels of the ions comprising the ion beam determine the depth of implantation of the ions into the wafer workpieces. High energy processes such as those used to form retrograde wells in semiconductor devices require implants of up to a few million electron volts (MeV), while shallow junctions may only demand energies below I thousand electron volts (1 KeV).
The continuing trend to smaller and smaller semiconductor devices requires a ion beam beam line construction which serves to deliver high beam currents at low energies. The high beam current provides the necessary dosage levels, while the low energy levels permit shallow implants. Source/drain junctions in semiconductor devices, for example, require such a high current, low energy application.
In high current and high energy implanters, semiconductor wafer workpieces are mounted near the periphery of a rotatable workpiece support. As the support rotates, the workpieces pass through the ion beam and are implanted with ions. When implanting wafers, if the angle of incidence of the ion beam (implantation angle) is perpendicular or normal to the surface of the workpiece, an effect called "channeling" has been found to occur. When channeling occurs, the ions of the ion beam pass into the crystal lattice structure of the semiconductor wafers and achieve greater penetration depth than is normally the case. The effective tilt angle (ETA) is defined as the angle between the ion beam and a ray extending perpendicularly from the surface of the wafer workpieces. An ETA=0 degrees defines a channeling implantation.
If channeling not desired, the effective tilt angle ETA is increased slightly, usually in the range of 1-10 degrees so that the ion beam beam line is not exactly perpendicular to the workpiece surface. This is accomplished by tilting the workpiece support with respect to the ion beam beam line direction.
In some implantation applications, channeling is useful. However, in such channeling applications, that is, ETA=0 degrees, implantation depth is very sensitive to implantation angle variation across the workpiece. As the implantation angle varies across the workpiece, the depth of ion penetration into the semiconductor wafer workpieces changes markedly.
If implantation depth is to be accurately controlled, the implantation angle must not change significantly over the surface of the wafer. In some applications, for example, in channeling implants the maximum allowable variation in the implantation angle is 0.2 degrees.
However, current art implanters wherein the workpiece support rotates and the workpieces lie flat on a flat workpiece support pad, a variation in the implantation angle of over 1 degree with a 300 millimeter (mm.) (30 cm.) diameter wafer workpiece at an ETA=0 degrees (channeling implant) is usual.
What is needed is a wafer support apparatus that minimizes the variation of implantation angle over a range of effective tilt angles ETA from 0 degrees (channeling implantation) and greater (non channeling implantation).